1. Field of the Invention
The invention relates generally to methods of and apparatus for blending materials and more particularly the invention relates to blending bulk materials including granular materials and to conveying such materials.
2. Discussion of the Art
Blenders used for processing bulk materials including such granular materials as animal feeds, chemical fertilizers or the like typically include apparatus for loading or charging the materials into the blenders, a blending or mixing chamber to perform the actual blending function, and further handling provisions. For example a conveyor may receive blended materials after their discharge from the blender or the materials may be fed through chutes into bagging operations. In many commercial bulk mixing operations the blended product, if not bagged, may be discharge into storage bins or discharged onto a vehicle to be transported to its intended place of use. Discharge conveyors, consequently may lift the blended product to substantial discharge heights, particularly when the blended materials are discharged by gravity into holding bins or into vehicle hoppers.
In all blending operations, blending efficiency and capacity are basic desired attributes. Most batch blenders are built to a predetermined size in terms of the largest batch of materials that can be processed at one time. Fertilizer blenders, for example, may be rated in terms of tons of blended material. Such blenders may have capacities of a relatively wide range, two to ten tons of material being within such range. It is of course desirable to blend any quantity of materials up to the maximum capacity of the unit with equal efficiency. Much effort has been exerted over the years to provide more efficient blenders.
Blenders which use rotating drums may historically have been the oldest types of blenders. In a blending process with early type blenders, proportioning the materials, such as by weighing them, feeding the materials from a staging hopper, such as a weigh hopper into the blender, blending them within the blender and then discharging and storing them as bulk or in weighed quantities involved separate operations. Later types of blenders sought to combine operations and streamline the blending process.
Vertical auger type blenders, for example, are sometimes referred to as "one-piece" blenders. Nevertheless, typical auger blenders feature independent feed-in power, mixing power, and discharge or unload power. Such independent power trains, though adding to the complexity of the blenders, are needed to satisfy simultaneous material loading power during the early cycle of the mixing process, and power for discharging the blended materials at the end of the mixing process.
Vertical auger type blenders typically feature a hopper with a downwardly conically converging base. A vertically operable auger is disposed centrally in the blending bin. The material becomes blended by being lifted centrally by the auger and by the distributing itself at the top surface of the batch of material in the blending bin. A screw type blender is typically discharged onto a conveyor at the bottom of the bin. Some grinding of the granular materials may take place in the screw conveyor and some separation of the materials may take place in a natural size grading process at the upper surface of the batch being blended. However, a more serious problem which may occur relates to a decrease of the mixing action on materials as their radial distance from the screw conveyor increases. Thus, as a batch size in the blender is increased, the efficiency of such a blender appears to decrease, thereby tending to increase the time required for fully blending a respective batch of the materials.
Other blenders referred to as paddle wheel blenders combine material weighing, hence proportioning, and blending operations to be performed in a single piece of equipment. Paddle wheel blenders provide a blending or mixing action by rotating paddles which extend from a typically horizontally disposed shaft. The rotating paddles are forced into the batch of material to be blended. The blending action becomes more thorough than that of auger type blenders. However, the blending operation also tends to become more power intensive than necessary. Also, a grinding of granular materials should be expected to occur throughout the blending cycle as a result of the paddle wheel action in the proximity of the inner surface of the blending drum.
The above mentioned, conventional rotating drum type blender does not generate such a grinding action within its blending chamber. A rotary drum is disposed to rotate about a substantially horizontal axis. Flights attached to the inside of the drum rotate with the drum and are thus more gentle on the materials therein. The flights lift and drop the materials to achieve the mixing action. On a particular variation of the rotating drum blenders, the axis of rotation of the drum is typically disposed at a slight incline. The drum has helically winding flights on the inside of the drum and a charging and discharging opening disposed at the higher of the two ends. A central opening leads into the drum to load or charge the materials to be blended. An annular inner wall between the central opening and the outer drum enables the helical flights disposed in the annular space between the inner wall and the outer wall to function as a screw type discharge conveyor. Thus, during the mixing process, the drum may be rotated in one direction, such that the annular screw conveyor functions to urge the materials out of the annular region of the drum and toward the mixing region in a main portion of the drum. To discharge the materials from the drum, the rotation of the drum is reversed and the helical flights function as a screw auger to discharge the blended materials from the drum. Since the rotating drum type blenders typically cause little or no grinding action on granular materials, dust levels from ground fines which are typically connected with blending of dry, granular materials are reduced. On the other hand, overmixing may segregate the blended materials according to different particle sizes. To the extent that different constituents may have respectively different average particle sizes, care must be exercised in selecting materials of similar particle size and in supervising the blending process so that the blended quality of the final product is not compromised.
As already suggested from the prior discussion, one of the disadvantages common to all known blenders relates to the relative complexity of the operations of handling and blending the materials. Handling equipment for transferring the blended materials from the blender to transport vehicles or storage bins becomes substantially as complex as the blending apparatus itself. Subsequent handling of the blended materials causes dust generation and often some small amount of material leakage during the transfer of the material from the blending apparatus to a conveyor. The complexity results from selectively activated conveyors, and from the need to provide housings or shielding for the materials from the elements and for minimizing material leakage and dust generation particularly during transfer processes, such as by conveyors.
Dust generation during the actual mixing process can typically be contained within the substantially enclosed blending chambers or drums. However, to the extent that the materials are fed into the blending chambers and are typically discharged from the blending chambers onto loading conveyors, airborne dust generation during the handling of the materials generally tends to remain a problem. The dust problem may be aggravated when the materials handled are chemicals like fertilizers, and clean air standards require extensive shielding to control the escape of dust.
With respect to blending chemical fertilizers, another problem relates to spillage or leakage of the materials from conveyors while the materials are handled outside of the blending chambers, as during loading and unloading the blender. A spillage can result from a material leakage during the discharge of material from the blending chamber onto a conveyor to transfer the blended material to the transport vehicle. Typically the leakage may be minor, but even then presents a problem when continuous and cumulative. Though conveyors are often encased by housings, loading hoppers at the lower ends of the conveyors typically cannot be totally sealed against conveyor belts to prevent leakage. The materials tend to bounce during acceleration and there may be a certain amount of carry back at the ends of a typical belt conveyor. Any amount of material leakage, however, is undesirable if not unacceptable.